DESCRIPTION (adapted from the application) We have recently discovered a novel homeobox gene that we have named Lbx2. This gene is homologous to the Drosophila ladybird genes and has been highly conserved through millions of years of evolution. In Drosophila and in mammals, ladybird genes play important roles in cell fate determination. Lbx2 is expressed in the developing urogenital ridge beginning at embryonic day 10.5. It is transiently expressed in many tissues derived from this structure, and expression subsequently is extinguished prior to birth. In preliminary experiments, we have identified the human homologue of Lbx2 and we have mapped the murine and human genes. Human LBX2 is located on chromosome 2pl2, very close to the region implicated in Alstrom. syndrome, a human disorder characterized by hypogonadism. We have characterized the Lbx2 promoter in transgenic mice and identified a region capable of recapitulating the urogenital expression pattern. We have also engineered a targeted mutation in Lbx2 in embryonic stem cells so that we will be able to inactivate this gene in the mouse. Our lab has successfully used CreLox technology to fate-map precursor cells expressing developmentally important transcription factors resulting in novel insights recently published or in press. Here, I propose to expand upon our preliminary results involving Lbx2 and to perform cell fate determination studies in addition to gene inactivation experiments. Specific aims are: to complete a more detailed expression analysis of Lbx2 during genitourinary development and to prepare antibodies to Lbx2; to identify tissue-specific regulatory elements within the Lbx2 promoter/enhancer and to use these elements to direct expression of Cre-recombinase in transgenic mice; to fate-map Lbx2-expressing precursors using Cre-reporter mice; and to inactivate Lbx2 using homologous recombination in embryonic stem cells in order to determine the function of Lbx2 during murine development.